Anti-lock brake control system

ABSTRACT

An anti-lock brake control system has a brake pressure modulator operated by a single solenoid controlled on-off valve which effects brake apply, brake release, and brake pressure hold modes. The hold mode is achieved by dithering the solenoid valve. An electronic circuit responsive to rotation of a braked wheel supplies signals to the solenoid valve to effect the various modes. The circuit includes a wheel speed recovery circuit for supplying to the valve during wheel recovery an operating signal for dithering the valve at a first level of wheel acceleration and for producing a brake apply signal above a higher value of wheel acceleration.

United States Patent Bowler et a1.

[ Apr. 17, 1973 32-54 CIRCUIT RESET I -wHEEL sPEEO LP RECOVERY CIRCUIT ANTI-LOCK BRAKE CONTROL FOREIGN PATENTS OR APPLICATIONS SYSTEM 1,914,765 10/1970 Germany ..303/2l BE [75] Inventors; Lauren L. Bowler, Bloomfield Hill 1,919,929 11/1970 Germany ..303/21 CF John L. Harned, Grosse Pointe Woods, both of Mich. Primary Examiner-Milton Buchler [73] Assignee: General Motors Corporation, Assmam Exammer stephen Kunm Attorney-Jean L. Carpenter, Paul Fltzpatrlck and DetroIt, Mich.

Warren D. H1" [22] Filed: Mar. 15, 1971 211 Appl. No.: 124,004 [571 ABSTRACT An anti-lock brake control system has a brake pres- 521 US. Cl. ..30s/21 P, 303/20, 303/61 sure modulator operated y a Single Solenoid [51] Int. Cl. ..B60t 8/12 trolled on-off valve which effects brake pp y, brake [58] Field of Search ..I88/ 181 A; 303/20, release, and brake pressure hold modes. The hold 303/61, 21; 324/162; 340/262 mode is achieved by dithering the solenoid valve. An electronic circuit responsive to rotation Of a braked [56] References Cited wheel supplies signals to the solenoid valve to effect UNITED STATES PATENTS the varlous modes. The Cll'Clllt Includes a wheel speed recovery circuit for supplying to the valve durlng 3,494,671 2/ 1970 Slavin et a1 ..303/21P wheel recovery an operating signal for dithering the 3,499,639 3/1970 p et valve at a first level of wheel acceleration and for 3,558,197 H1971 LUBCk 6t 31 .303/2l BE producing a brake pp y Signal above a vaue of wheel acceleration.

1 Claim, 4 Drawing Figures AZLVLW, T VEHICLE WHEEL l'l'ACi'iOh/lli'l'E1 DRWE l0 v L Q r L F WHEEL RELATIVE HYSTERESIS F CONTROL} INFORMATION VELOCITY SWITCH LOCIC CIRCUIT \NTEGRATOR 16 14 .1 i I IHRESHOLO w E i C (j C L HT w? i E 79W ryz TO AMgPHERE I WHEEL HOP E7 SUPPRESSION CIRCUIT PATENTEDAPR I 71975 SHEET 1 or 2 TACHOMETER VEHICLE WHEEL DRIVE WHEEL INFORMATION CIRCUIT RELATIVE VELOCITY INTEGRATOR HYSTERESIS SWITCH LOGIC THRESHOLD LOGIC CIRCUIT WHEEL HOP SUPPRESSION CIRCUIT CIRCU IT RESET [i COEFFICIENT 66 .SELECT CIRCUIT WHEEL S PEE D RECOVERY CIRCUIT TO ATMOSPHERE VACUUM SOURCE VALVE CONTROL I INVENTORS [aarezzfi Bow/er "John L. Heme! ATTORNEY PATENTEU 1 7 5.

SHEET 2 OF 2 INVENTORS flaurezz L. Bow/er BY Jc r/m L Harnea ATTOR N EY ANTI-LOCK BRAKE CONTROL SYSTEM This invention relates to an anti-lock brake control system and particularly to one achieving a brake pressure hold mode requiring only a signal on-off solenoid valve.

It has previously been proposed as in the patent application of John L. I-Iarned, Ser. No. 754,252, now U.S. Pat. No. 3,554,612 to provide an anti-lock brake control having a brake pressure modulator with two solenoid controlled valves, one to effect brake pressure apply and release and the other to effect a pressure hold which maintained a constant brake pressure whenever that valve was energized. The advantage of the hold mode is that it improves overall system performance when considering operation over the entire range of conditions. The requirement of a second solenoid control valve to achieve the hold mode, however, adds an expense to the system which the present invention eliminates.

It is therefore an object of the invention to provide an anti-lock brake control having a pressure hold mode and which requires only a single on-off solenoid valve.

Another object of the invention is to provide a circuit responsive to a first predetermined level of positive wheel acceleration to produce an oscillating signal to achieve a hold mode.

It is another object of the invention to provide a circuit responsive to a second value of positive wheel acceleration during wheel speed recovery for producing a brake apply signal when the second value of acceleration has been exceeded.

The invention is carried out by providing a circuit responsive to rotation of a braked wheel for sensing incipient wheel lock-up which controls a brake pressure modulator having a single on-off valve directly controlled by the circuit for relieving brake pressure when incipient wheel lock-up is detected, and particularly including a wheel recovery circuit sensitive to positive wheel acceleration for providing an oscillating output signal at a predetermined first value of acceleration to effect a pressure hold mode by dithering the on-off valve. The invention further contemplates a provision in the wheel speed recovery circuit sensitive to a second higher value of wheel acceleration to effect brake reapplication via the on-off valve.

The above and other advantages will be made more apparent from the following specification taken in conjunction with the accompanying drawings wherein like reference numerals refer to like parts and wherein:

FIG. 1 is a block diagram of an anti-lock brake control system coupled with a schematic diagram of a hydraulic brake system with a brake pressure modulator according to the invention;

FIG. 2 is a schematic diagram of the control circuit of FIG. 1 according to the invention;

FIG. 3 is a graphical representation of vehicle speed, wheel speed and brake pressure to illustrate the operation of the control system according to the invention as applied to a high coefficient surface; and,

FIG. 4 is a graphical representation of vehicle speed and wheel speed to illustrate the operation of the control system as applied to a low coefficient surface.

The system in which this invention is utilized follows the well-established principle of sensing incipient wheel lock-up when brake pressure is applied to vehicle brakes, then relieving the brake pressure until the wheel accelerates enough to be out of danger of locking and then reapplying the brake pressure. This cycle is repeated as necessary to achieve the desired braking action. This system is particularly well adapted to practice the control principle of the extremal type in which brake pressure sufficient to cause a substantially increased wheel slip is released to permit wheel acceleration and consequent decreasing wheel slip. The brake pressure is then controlled to permit wheel acceleration and therefore a decrease in wheel slip while maintaining a brake torque on the wheel until the wheel acceleration ceases. The brake apply pressure then again is increased to cause wheel deceleration. This extremal type of control is more fully set forth in the U.S. Pat. No. 3,441,320 to D. M. Flory.

The system further employs an improved version of the Inertia Wheel Velocity Reference Principle which is fully set forth in the aforesaid U.S. Pat. No. 3,554,612. States briefly, the principle involves measuring the velocity of a braked wheel and subtracting therefrom a simulated vehicle velocity (or optimum wheel velocity) and utilizing the difference as a criterion of when the wheel brake should be released and reapplied for most effective control.

Referring to FIG. 1 for a further description of the system, a vehicle wheel drive 10 is provided as an information source for the system. The vehicle wheel drive 10 may be a wheel per se, a propeller shaft driving a plurality of wheels or any other vehicle member having a velocity or rotation proportional to wheel velocity. A tachometer 12 such as a well-known toothed wheel variable reluctance electromagnetic transducer is driven by the wheel drive 10 and provides an alternating signal having a frequency proportional to the wheel speed on line 14 and leading to a wheel information circuit 16. The wheel information circuit prepares a DC voltage or current proportional to wheel acceleration on lines 18 and 20. The wheel acceleration signal on line 18 is compared with an appropriate threshold signal on line 22 which is prepared by a threshold logic circuit 24 and the difference between the signals is integrated by a relative velocity integrator 26 to produce a relative velocity error signal on line 28, which signal is utilized by a hysteresis switch 30 to signal via line 32 a valve control logic circuit 34 to release brake pressure.

A brake pressure modulating system comprises a combination pneumatic and hydraulic system. The pneumatic portion comprises a tube 36 vented to atmosphere at one end and connected at the other end to a vacuum source 38 which may be the vehicle engine manifold. The tube 36 contains in series a restricting orifice 40 near the vent to atmosphere, a pressure accumulator 42, a normally closed electrically operated release valve 44 controlled by an output signal on the conductor 46 from the valve control logic circuit 34, and a second restricting orifice 48. A branch tube 50 is connected to the tube 36 at a point between the valve 44 and the orifice 48 and leads to a pressure modulator 52. Another pneumatic tube 54 connects the modulator 52 with the vacuum source 38. The hydraulic portion of the system includes a manually operated conventional master cylinder 56 for applying brake pressure to conventional vehicle brakes 58 through a first supply line 60 to modulator 52 and a second supply line 62. The structure and operation of the modulator 34 is fully described in the above-mentioned Flory patent and no further description herein is necessary except to point out its function.

Normally, with valve 44 closed, vacuum pressure from the source 38 is applied through the tubes 50 and 54 to both sides of the modulator 52 and in this condition, the modulator transfers the brake pressure from the master cylinder 56 to the brakes 58 without any modulation. However, when the valve 44 is opened to supply air pressure from the atmosphere to the modulator 52 through the tubes 36 and 50, the modulator will first isolate the brakes 58 from the master cylinder 56 and will then relieve pressure from the brakes 58 to a degree determined by the amount of air pressure supplied through the tube 50. The function of the accumulator 42 is to permit a rapid release of brake pressure for a short time after the valve 44 is opened. Thereafter brake pressure will decrease more slowly as determined by the flow of air through the orifice 40. When the valve 44 is closed, the brake pressure increases toward master cylinder pressure at a controlled rate determined by the air flow through the orifice 48.

The threshold logic circuit 24 senses the operating characteristics of the system and selects the appropriate threshold to be biased against the incoming wheel deceleration signal. Before brake pedal application, the circuit reset 64 signals on line 65 and l2g threshold. This masks any random noise signal from momentarily triggering the anti-lock control, for example, when controlled wheels have been overspeed during vehicle acceleration. The remaining thresholds are applied depending on operating mode, road surface coefficient, and impending wheel hop during rough road operation.

The coefficient select circuit 66 senses the wheel acceleration signal on line to determine whether the wheel speed recovers in excess of 4gs. If it does, then a 2g threshold is applied via line 67 to the threshold logic circuit 24during the next brake apply cycle. Otherwise, a lg rate is selected. The 4g wheel speed recovery rate discriminates between most road surfaces and the very slippery icy surfaces which require a lower threshold for proper operation. The coefficient select circuit 66 also senses the operation of the hysteresis switch 30 via the line 68 to reset the lg threshold whenever the hysteresis switch calls for a brake release. The coefficient select circuit 66 is also actuated by the circuit reset 64 via line 69 to further increase the threshold when the brake pedal is not actuated.

A wheel hop suppression circuit 70 senses wheel accelerations in the wheel hop frequency range by filtering the acceleration signal on line 20 with a bandpass filter. When this circuit senses an acceleration of 2gs or more at the wheel hop frequency, then it signals the threshold logic circuit 24 via line 72 to apply an additional threshold of 1.5gs during the brake apply mode. The hysteresis switch also unlocks the wheel hop suppression circuit 70 via line 68 to insure that the 1.5g threshold is off during release. No more than lg is applied to the relative velocity integrator 26 during release. 7

The wheel speed recovery circuit 74 is responsive to the acceleration signal and provides the operating sense to appropriately control the valve control logic 34 via the line 76 during the wheel speed recovery phase of the cycle, which controls the hold mode and the 2g apply technique. The 2g apply technique allows brake pressure to be reapplied during wheel speed recovery whenever the acceleration of the wheel exceeds approximately 2gs. The wheel speed recovery circuit also disables the hysteresis switch 30 via line 77 to prevent brake application when the wheel acceleration exceeds 0.5g.

FIG. 2 illustrates the details of the control circuit. Wheel Information Circuit The wheel information circuit 16 has as an input from the tachometer 12, on line 14, a train of pulses having a frequency proportional to the sensed wheel speed. The pulses are fed into a high gain squaring amplifier 80 having an input resistor 82 and a feedback resistor 84 to produce an output train of square pulses with constant amplitude and edge slope. A differentiator comprises a capacitor 86, a resistor 88 and a diode 90 in series with the output of the amplifier 80 and a diode 92 between the capacitor 86 and ground, and produces a negative pulse of constant area for every square pulse input. The duration of the pulse is less than the period of the input pulses at maximum vehicle speed. This prevents an overlap of pulses which would introduce non-linearities. The negative pulses from the differentiator are then filtered by an active first order low pass filter which comprises an operational amplifier 94 having a feedback capacitor 96 and a feedback resistor 98. A feedback diode 100 in conjunction with a biasing voltage divider network including resistors 102 and 104, sets a low speed threshold that must be overcome by the negative pulses before a positive voltage will appear at the output of the amplifier 94. This provides a low speed cutout feature for the circuit. The resistor 104 is connected at opposite ends to a 7.5v and +7.5v DC powersupplyl The output of the amplifier 94 is a positive voltage proportional to wheel speed. This voltage is then differentiated by two circuits. The first is a noninverting passive differentiator comprising a series connected capacitor 106 and resistor 107 which produces a current on line 18 proportional to wheel acceleration. The second differentiator is also a passive differentiator comprising capacitor 110 and a resistor 112 serially connected between the output of the amplifier 94 and the input of non-inverting operational amplifier 114. A low pass filter comprises feedback capacitor 1 l6 and feedback resistor 118 across the amplifier 114 to smooth out the ripple. Threshold Logic Circuit The threshold logic circuit 24 includes an array of input resistors 120, 122, 124 and 126, each having one end connected to the common line 22. The other ends of the resistors are connected to the outputs of the wheel hop suppression circuit 70, the coefficient select circuit66, the circuit reset 64 and the +7.5v DC power supply respectively. The resistor values are chosen to provide threshold bias currents to line 22 according to the desired circuit functions as recited above. The resistor 126 provides the minimum deceleration threshold of l g The other thresholds when applied are additive and will increase the threshold according to their respective values. Relative Velocity Integrator The relative velocity integrator circuit 26 senses the wheel acceleration signal on line 18 and the threshold bias on line 22. The integrator includes an operational amplifier 128 having a feedback circuit including a gain resistor 130 in series with an integrating capacitor 132, a resistor 134 between ground and the junction of the resistor 130 and capacitor 132, and a diode 136 across the resistor 130. These components give the amplifier a dual integration gain. A high gain exists when the amplifier output is negative and the diode 136 is blocking. However, as the amplifier output moves into its positive range, the diode 136 conducts to shunt the gain resistor 130 to lower the gain. This feature offers a sensitive range for triggering system switch points for control purposes and a lower gain range for storing excessive relative wheel speed buildup. A pair of oppositely poled Zener diodes 137 and 140 are serially connected across the amplifier 128 to limit the amplifiers output short of its saturation values. In operation, when the system is not controlling, that is, when the current on line 18 flowing from the amplifier 128 does not exceed the threshold current on line 22, the output of the amplifier 128 is slightly above negative saturation. As the wheel deceleration increases to a value exceeding the threshold, then the output of the amplifier 128 increases, i.e., becomes less negative to signify the velocity error or the difference between the wheel velocity and the simulated vehicle velocity.

Hysteresis Switch The hysteresis switch 30 is operated by the output of the relative velocity integrator 26. This switch includes logic NOR gates as does much of the remainder of the circuit to be described. The gates switch at, say, 0.7v, so that when both inputs to a NOR gate are low (below 0.7v), then the gate output must be high (above 0.7v). If either or both inputs to a NOR gate are high, then the output will be low. NOR gates 142 and 144 are connected to form a set-reset flip-flop. The flip-flops inputs a and c respond to different relative velocity output levels of the amplifier 128. Input a to NOR gate 142 is connected through a resistor 146 to the output of the amplifier 128 on line 28. An NPN transistor 148 has its base connected through a resistor 150 to the line 28 and its emitter is connected to the midpoint of a voltage divider comprising resistors 152 connected between a 7.5v DC source and ground. The collector is connected through series resistors 154 and 156 to +7.5v. The junction point of resistors 154 and 156 forms an input 2 to a NOR gate 158 which has its other input grounded, and therefore acts simply as an inverter. The output of the gate 158 comprises an input i to a NOR gate 160 which has its other input signal h from the wheel speed recovery circuit 74. The output of the NOR gate 160 forms the input c to the NOR gate 144 while the other input of the NOR gate 144 is the signal d from the output of the NOR gate 142. To complete the flip-flop circuit, the output of the NOR gate 144 comprises an input b of NOR gate 142. Initially, when the voltage on line 28 is negative, transistor 148 is off and input 0 to gate 144 is high, assuming that the input h to gate 160 is low, as it will be during periods of wheel deceleration. Thus signal b will be low, and, since input a is also low, signal d will be high. Circuit values are so selected that when a velocity error of one-half ft. per second is accumulated by the relative velocity integrator, the transistor 148 starts to conduct so that signal 1 goes low, signal 1' goes high and signal c goes low. However, since signal d is high, the gate 144 will not change state until input a turns high. Input a remains low until 4 ft. per second velocity error has been accumulated by the relative velocity integrator, at which time the line 28 becomes positive and input will turn high. This switches signal d low which allows b to turn high. The flip-flop thus changes state and will not change back until the voltage on line 28 returns to the one-half ft. per second switch point that turns off transistor 148 to switch input c to high. Thus, the hysteresis switchs output d remains high until a 4 ft. per second relative velocity is reached, then it switches low until the velocity error diminishes to less than one-half ft. per second. Even then, the output d will remain low until the signal h goes low whereupon d switches high. The output a' is connected via line 32 to the valve control logic 34.

Valve Control Logic The valve control logic 34 consists of a buffer amplifier 162 which is an inverting current amplifier having a single input supplied by a diode 164 connected to signal d on the line 32 and by a diode 166 connected with a signalj on line 76 from the wheel speed recovery circuit. If both inputs d andj are low, then the amplifier 162 will be turned on to energize the normally closed solenoid valve 44 and release brake pressure. If, however, either the input d or j is turned high, then the amplifier 162 will be turned off to close the solenoid valve. The circuits described to this point relative to FIG. 2 are sufficient to control a basic anti-lock control system. However, to insure satisfactory system performance under virtually all possible operating conditions, the remaining circuits including the wheel speed recovery circuit 74, the wheel hop suppression circuit 70, the coefficient select circuit 66, and the circuit reset 64 are employed.

Wheel Speed Recovery Circuit The wheel speed recovery circuit 74 accomplishes two objectives: (1) It reduces the complexity and cost of the hydraulic modulator by making a hold mode solenoid valve unnecessary, and (2) it reduces the total response characteristics necessary for good anti-lock control performance by instituting brake apply during the wheel speed recovery when the wheel accelerates in excess of 2gs. The wheel recovery logic circuit eliminates the need for a separate hold valve by replacing its function with a rapid switching of the release valve 44 between release and apply to modulate a nearly constant brake pressure.

NOR gates 168 and 170 are connected as a set-reset flip-flop circuit which constitutes a hold switch. Signal d from the hysteresis circuit forms the reset input and is applied to gate 168. Input h is applied to the gate 170 for the set input and is produced by a voltage level detector comprising a diode 172 connected between the line 20 and the input h so that when the positive acceleration signal is below 0.5g, the diode 172 does not conduct and the input h is low. However, when the acceleration exceeds that value, h becomes high. The output f of the flip-flop circuit and the input h comprises inputs to a NOR gate 174. The output g of the gate 174 provides an input to a multivibrator which includes NOR gates 176 and 178. The output j of gate 176 is connected through a capacitor 180 to an input n of the gate 178. The input n is also connected to ground through a resistor 182. Similarly, the output k of the gate 178 is connected through a capacitor 184 to an input m of the gate 176, which input is also connected to ground through a resistor 186. The other input x of the gate 178 is connected through a Zener diode 188 to the diode 172. The Zener diode is selected to break down at 1.5gs so that it acts as a second voltage level detector. Thus, when wheel acceleration exceeds 2gs, the voltage on line 20 will be sufficient to cause both the diode 172 and the Zener diode 188 to conduct to switch the input x to a high level, whereas at lower values of acceleration, the Zener diode 188 will not conduct and the input will be at a low level. The outputj of the gate 176 is connected through an output resistor 190 to the line 76.

The multivibrator has three operating modes. When the input g is high, the outputj is low to enable a brake release if the signal d is also low. When the input g is low, and the input 1: is high, thereby holding the signal m low, then the output j is high causing a brake pressure application. When, however, the input 3 is low, and the input x is also low, then the multivibrator will serve as an osciliator supplying rapidly alternating high and low pulses. The oscillation mode is a result of pulse decay timing by the capacitors 180 and 184 and resistors 182 and 186. Since m is normally low and g is low, outputj will be high turning signal n high momentarily. As the charge on the capacitor 180 decays, n will become low and signal k will switch high to momentarily turn the signal m high which in turn turns the output j low until the capacitor 186 discharges and m turns to its low level causing j to switch high again to cause repetition of the cycle.

in operation of the wheel speed recovery circuit, prior to brake release, the signal d is high causing the signalfto be low. Since the input 11 must be low during brake application and wheel deceleration, then the signal e is high. Since the signalsfand h are low, then the signal g will be high and the signal j will be low to thereby permit brake release when signal at is switched low by the hysteresis switch. However, at brake release when d goes low, the flip-flop circuit does not change state since the signal e is high andfremains low. When, however, after brake release the wheel accelerates to a value above 0.5gs, the signal 11 goes high to cause g to go low, causing j to oscillate to automatically turn the valve 44 on and off to hold the brake pressure at a relatively constant value. When, however, the acceleration signal online 20 exceeds 2gs, the input x goes high causing the outputj to stay high to effect brake application. Should the wheel acceleration then drop below 2gs, the input x will drop to a low level and the outputj will return to its oscillating hold condition. When the wheel recovers sufficient speed that the velocity error drops to one-half ft. per second, the transistor 148 in the hysteresis switch turns off so that signal z at gate 158 goes high and its output i goes low. However, if the wheel is still accelerating above a value of 0.5g, the signal It will still be high and the signal will remain low so that the flip-flop 142,144 does not change state and the signal d remains low whereby the wheel speed recovery circuit is the controlling agent. When, however, the wheel acceleration drops below a 0.5g and signal It goes low, signal c will go high permitting signal d to go high to close the valve 44 and reapply the brakes. Wheel Hop Suppression Circuit On rough roads the vehicle suspension system is excited at its wheel hop frequency of about 10 Hz and large wheel acceleration amplitudes are generated, particularly during braking. These large acceleration excursions may cause premature system operation which releases brake pressure to assume a value below that called for by the driver or below that allowed by the road surface coefficient friction value for maximum vehicle deceleration. The wheel hop suppression circuit senses the transient signals caused by the wheel hop and renders the system less sensitive to such signals by increasing the acceleration threshold by an additional 1.5g. The circuit includes a bandpass filter comprising capacitors 192 and 194 and a resistor 196 connected to line 20 to sense acceleration signals. The filter passes those signals having frequency in the range of 10 Hz; A Zener diode 198 in series with the filter is selected to conduct those signals in excess of 2g to provide an input a to a NOR gate 200 having its other input grounded. The output v of the NOR gate 200 provides an input to a NOR gate 202 having an output w on line 72. The second input to the NOR gate 202 is the signal b from the hysteresis switch 30. Signal b is high during brake release and low at brake apply. Thus, during brake release, the output w is always low. When, however, during brake apply, the Zener diode 198 senses a transient acceleration in excess of 2gs, the input u is high, the signal v will be low so that the signal w will be high. The signal w is connected to the resistor in the threshold logic circuit 24 to increase the threshold bias by LSgs. Thus the system is made less sensitive to wheel hop signals. When, however, there are no such large wheel hop signals on line 20, the input u of NOR gate 200 will be low and the output w will likewise be low so that the threshold bias is not effected by the wheel hop suppression circuit. Coefficient Select Circuit On road surfaces where the friction coefficient exceeds 0.3, wheel acceleration during wheel speed recovery will always exceed 4g. On ice covered surfaces, the peak wheel acceleration will always be less than 4g. On surfaces where the friction coefficient falls between 0.15 and 0.3, the occurrence of peak wheel accelerations greater or less than 4g will depend on the specifics of the friction slip relationships. When peak wheel acceleration does not reach a 4g level, it indicates either that the peak tire torque value is only slightly greater than or equal to the locked wheel tire torque or that the friction slip curve slope following the peak has a small negative value. Either of these situations degrades extremal stability and could cause premature wheel lock-up. System extremal stability can be increased by decreasing the deceleration threshold bias to the relative velocity integrator 26. The effect of the coefficient select circuit 66 is to alter the threshold bias in accordance with the coefficient of friction between the tire and road as reflected in the acceleration signal during wheel speed recovery. The circuit includes an acceleration signal level detector comprising a Zener diode 204 and a diode 206 connected serially to the line 20. These elements in combination conduct at a signal level of 4gs to provide signal r to one input of a NOR gate 208. That gate, along with NOR gate 210, provides a flip-flop circuit such that the output s of the gate 208 serves as an input to the gate 210 and the output I of gate 210 is an input for the gate 208. The other input signal b to gate 210 is developed by a capacitor 212 connected from the line 68 carrying signal b from the hysteresis switch. The input line b is connected to ground through a resistor 214. The output s of the flipflop forms one input to a NOR gate 216 and the signal b serves as the other input. The output y of the gate 216 is connected via line 67 to the resistor 122 in the threshold logic circuit 24.

During the brake release phase, b is high and the output y is always low. When the signal b first goes high, at release, the signal is differentiated by the capacitor 212 so that the signal b goes high momentarily to set the flip-flop. That is, when b goes high, I will go low and since r is always low, at the time of brake release, s will go high. During the brake release phase, if the road coefficient is low, the wheel acceleration will not exceed 4gs and the signal r will remain low so that the signal y will remain low and will therefore not contribute to the threshold bias during the subsequent antilock cycle. If, on the other hand, the coefficient is high, the input r will go high sometime during the wheel recovery phase so that s will go low to reset the flipflop. That is, 1 will go high and s will be held low. Then, during the apply phase of the subsequent cycle, when the signal b goes low, the signal y will go high to increase the threshold bias an amount equivalent to lg. The signal y will remain high until brake release whereupon signal b goes high to turn y low and b will go high momentarily to set the flip-flop and return the signal s to a high level. A diode 218 connected to an input of the NOR gate 208 carries a signal q. As will be seen, when the vehicle brakes are off, the signal q will be high to reset the flip-flop to send the signal y high as long as the brakes are off and during the apply phase of the initial anti-lock cycle.

Circuit Reset The circuit reset 64 comprises a NOR gate 220 having one input grounded and the other input connected through voltage dividing resistors 221 and 223 to ground and to conductor 225 carrying the brake light signal. The output q of the NOR gate 220 is connected to the diode 218 as previously described and also via line 65 to the resistor 124 in the threshold logic circuit 24. The resistor 124 is selected to provide a g threshold bias current when q is high. When the vehicles brakes are off and the vehicle brake lights are not illuminated, the inputs to the NOR gate 220 are low and the output q is high so that the 10g threshold bias is applied. As mentioned above, when the signal q is high, the signal y is also high to turn on its corresponding lg threshold bias. These biases added to the constant lg bias from resistor 126 provide a total threshold bias of l2gs which effectively desensitizes the relative velocity integrator 26 so that extraneous wheel deceleration signals will not activate the hysteresis switch. When, however, the brakes are first applied and the brake lights are illuminated, the conductor 225 is energized to provide a high signal to an input of the NOR gate 220 which turns the signal q low and removes the 10g threshold bias. Then the threshold bias is set at 2gs in readiness for the initial anti-lock cycle.

iii

System Operation The overall operation of the system is best understood by reference to FIGS. 3 and 4. FIG. 3 is a graph showing vehicle speed on line 222, wheel speed on line 224, reference signal on line 226, and the brake pressure on line 228. The graph illustrates typical antilock operation on a high coefficient road surface. The reference signal 226 is an imaginary signal not actually existing in the control circuit, however, its slope represents the threshold bias signal. The graph assumes initially no brake pressure is applied and the vehicle speed and wheel speed are constant. At time t brake pressure is first applied and the vehicle and wheel speeds decrease. The reference signal 226 initially has a slope of 2gs as described above. The relative velocity integrator 26 provides an output on line 28 which is proportional to velocity error or the vertical distance between the curves 224 and 226. When, at time t,, this distance equals 4 ft. per second, the hysteresis switch 30 is actuated to switch the signal d low to effect brake release, whereupon the brake pressure begins to fall. At the same time, the signal b goes high to turn off the signal y to reduce the threshold bias thereby changing the reference signal 226 to a slope of lg. At time t the wheel has accelerated to 0.5gs and this acceleration is sensed by diode 172 in the wheel speed recovery circuit to raise the signal h to a high level causing outputj to oscillate holding the brake pressure constant until time t Then the wheel acceleration reaches 2gs whereupon the signal level detector 188 conducts and causes the output j to remain high to effect brake reapplica tion. At time t, the wheel acceleration has decreased to a value below 2gs whereupon the outputj again oscillates to hold the brake pressure at a constant value until time when the wheel acceleration has dropped below a 0.5 gs level and the wheel is considered to have recovered in speed. Prior to that time, the output of the integrator 26 has decreased to one-half ft. per second and the signal a has gone low. When the signal h goes low at time i the signal d goes high to cause brake reapplication. Then brake pressure continues to increase at a controlled rate until time t in the subsequent anti-lock cycle when the relative velocity error again signifies incipient wheel lock-up and the brake pressure is then again released. Then the cycles repeat until the vehicle is brought to a stop or the brake pressure is manually released.

During wheel recovery, the wheel acceleration exceeded 4gs signifying a high coefficient surface. This acceleration is sensed by the coefficient select circuit and the 2g threshold is applied during the apply phase of the next cycle so that the reference signal 226 is like that of the first cycle. This is contrasted to FIG. 4 of the drawing which illustrates the operation of an anti-lock brake control on an icy surface. The lines 222, 224 and 226 represent the same parameters as in FIG. 3. The brake pressure is not shown in FIG. 4 since it is generally similar to that of FIG. 3. As before, in the initial cycle, the imaginary reference signal 226 has a 2g slope until time t when the brakes are released and then the slope changes to lg. However, during wheel recovery, the wheel acceleration does not exceed 4gs so that the coefficient select circuit 66 is not reset and the signal y remains low. Consequently, for the subsequent cycle, the slope of reference signal 226 remains at lg resulting in a more sensitive detection of incipient wheel lock-up so that brake release at time t occurs earlier than if the slope were at 2gs. The system extremal stability is therefore increased.

The embodiment of the invention described herein is for purposes of illustration and the scope of the invention is intended to be limited only by the following claims.

it is claimed:

1. An anti-lock brake control system for a wheeled vehicle comprising means responsive to the rotation of a braked wheel for detecting an incipient wheel lock-up condition and for thereupon producing a brake release signal,

means for generating an acceleration signal according to acceleration of the braked wheel,

wheel recovery means enabled by the brake release signal including a multivibrator, a first signal level detector responsive to the acceleration signal and brake pressure modulating means responsive to the brake release signal for relieving brake pressure when incipient wheel lock-up is detected to allow positive wheel acceleration, responsive to the oscillating output signal for holding brake pressure at a relatively constant value during the predetermined range of wheel acceleration, and responsive to the brake apply signal or reapplying brake pressure when the "wheel acceleration exceeds the second value.

12323? UNITED STATES PATENT OFFICE CEETIFECATE OF CORRECTION Patent No. 3,727,992 Dated April 17, 1973 hvemofls) Lauren L. Bowler and John L. Harned error appears in the above-identified patent It is certified that corrected as shown below:

and that said Letters Patent are hereby i- Column 2, line 20, change "States" to Stated Column 6, line 7, after "input" insert a 1 Column 12, line 15, change "or" to for Signedand sealed this 1st day of January 19714..

(SEAL) Attest:

EDWARD M.ELETCHER,JR. RENE D. TEGTMEYER Attesting Officer Acting Commissioner of Patents 

1. An anti-lock brake control system for a wheeled vehicle comprising means responsive to the rotation of a braked wheel for detecting an incipient wheel lock-up condition and for thereupon producing a brake release signal, means for generating an acceleration signal according to acceleration of the braked wheel, wheel recovery means enabled by the brake release signal including a multivibrator, a first signal level detector responsive to the acceleration signal and connected to the multivibrator for effecting a rapidly oscillating multivibrator output above a first predetermined level of wheel acceleration, and a second signal level detector responsive to the acceleration signal and connected to the multivibrator for effecting a brake apply signal from the multivibrator, and brake pressure modulating means responsive to the brake release signal for relieving brake pressure when incipient wheel lockup is detected to allow positive wheel acceleration, responsive to the oscillating output signal for holding brake pressure at a relatively constant value during the predetermined range of wheel acceleration, and responsive to the brake apply signal for reapplying brake pressure when the wheel acceleration exceeds the second value. 